What are the advantages and disadvantages of different diversity schemes for fading channels?

1 Spatial diversity

Spatial diversity uses multiple antennas at the transmitter, the receiver, or both to create independent signal paths that can avoid or mitigate fading. Spatial diversity can be implemented in different ways, such as beamforming, multiple-input multiple-output (MIMO), and cooperative communication. The main advantage of spatial diversity is that it can significantly enhance the capacity and performance of wireless systems by exploiting the spatial dimension. However, spatial diversity also has some drawbacks, such as increased complexity, cost, and power consumption of the hardware, as well as the need for accurate channel estimation and feedback.

2 Frequency diversity

Frequency diversity uses multiple carrier frequencies or bandwidths to transmit the same or different information over different frequency channels. Frequency diversity can benefit from the fact that fading is frequency-selective, meaning that different frequencies experience different levels of attenuation and distortion. Frequency diversity can be achieved by techniques such as frequency-hopping spread spectrum (FHSS), orthogonal frequency-division multiplexing (OFDM), and cognitive radio. The main advantage of frequency diversity is that it can increase the robustness and flexibility of wireless systems by exploiting the frequency dimension. However, frequency diversity also has some challenges, such as the limited availability and regulation of spectrum resources, the increased susceptibility to inter-channel interference, and the requirement for synchronization and coordination among different frequency channels.

3 Time diversity

Time diversity uses multiple time slots or intervals to transmit the same information over the same channel. Time diversity can take advantage of the fact that fading is time-varying, meaning that the signal strength changes over time due to the relative movement of the transmitter, receiver, or scatterers. Time diversity can be realized by methods such as time-hopping spread spectrum (THSS), time-division multiplexing (TDM), and automatic repeat request (ARQ). The main advantage of time diversity is that it can improve the reliability and efficiency of wireless systems by exploiting the time dimension. However, time diversity also has some limitations, such as the increased latency and overhead of transmission, the reduced throughput and bandwidth efficiency, and the dependence on the coherence time of the channel.

4 Polarization diversity

Polarization diversity uses multiple polarization states or modes to transmit the same or different information over the same channel. Polarization diversity can leverage the fact that fading is polarization-dependent, meaning that different polarization states experience different levels of attenuation and phase shift. Polarization diversity can be implemented by using polarized antennas, such as linear, circular, or elliptical antennas, or by using polarization multiplexing or modulation schemes. The main advantage of polarization diversity is that it can enhance the diversity gain and spectral efficiency of wireless systems by exploiting the polarization dimension. However, polarization diversity also has some drawbacks, such as the increased complexity and sensitivity of the antenna design and alignment, the potential degradation due to polarization mismatch or cross-polarization interference, and the influence of the polarization characteristics of the channel and the environment.